In the semiconductor electronic devices that process digital signals and analog signals at the same time, semiconductor devices processing the digital signals and semiconductor devices processing the analog signals are separately fabricated and then are assembled together. As the semiconductor electronic devices are scaled down, techniques for processing the digital signals and the analog signals within a single semiconductor device have been developed.
Bluetooth modules or RFID modules are representative semiconductor devices having both digital signal processing circuits and analog signal processing circuits. These modules include digital circuits (e.g., memory, arithmetic unit, etc.) and RF analog circuits (e.g., RF amp, PLL, antenna, etc.) within a single semiconductor package. These modules are called a mixed signal system. In a single semiconductor package, a plurality of semiconductor devices and a plurality of passive circuits manufactured by various processes are integrated into a single system. Thus, the semiconductor package is typically known as a system-in-package (SiP).
In the system-in-package processing the mixed signals, a digital circuit part and an analog circuit part may share a common power plane and a ground plane parallel to each other, and may use them separately. In any case, the two circuit parts are coupled directly or indirectly through various electromagnetic mechanisms. An issue is a wideband switching noise that is generated by a switching operation and clock signal of the digital circuit part and is inherently propagated to the analog circuit part. The power plane/ground plane may be considered as a kind of a parallel plate waveguide. A plurality of vias formed in the power plane/ground plane operates as an antenna receiving the switching nose. Because the switching noise has a wideband, it overlaps an analog signal band at which the analog circuit part operates. In addition, because the analog circuit is very sensitive to the switching noise, it is very important to suppress the switching noise.
Various approaches to reduce the switching noise when the parallel plate power plane/ground plane have been developed. Examples of the approaches include a method for suppressing resonance generated in a cavity between two plates, for example, a method for attenuating RF signals using absorbent or loss component, and a method for dividing a power plane. However, these methods are effective only to an electromagnetic wave having a band of a few hundreds MHz and a limited directionality and within a restricted region.
An electromagnetic bandgap (EBG) structure has been developed to suppress a surface current generated in an RF analog device. The EBG structure is inserted between the power plane and the ground plane and operates as an RF bandstop filter. The EBG structure is very effective to an electromagnetic wave of GHz having a planar omnidirectional characteristic and extends along the SiP and can be implemented at a low cost.
FIG. 1 is a perspective view of a conventional EBG structure. Referring to FIG. 1, the conventional EBG structure 10 includes a power plane 11, a ground plane 12, and an EBG layer 13. The power plane 11 and the ground plane 12 are arranged parallel to each other, and the EBG layer 13 is embedded between the two planes 11 and 12. The EBG layer 13 is connected to one of the two planes 11 and 12 through vias 14. In the case of FIG. 1, the EBG layer 13 is connected to the ground plane 12. The EBG layer 13 is divided into cells that are repetitively arranged at constant periods. The via 14 connects the EBG layer 13 to one of the two planes 11 and 12 at each cell. A dielectric having a predetermined permittivity is filled between the EBG layer 13 and the two planes 11 and 12. Because a low-temperature co-fired ceramic (LTCC) has a frequency stable permittivity property and a low loss, it is widely used as the dielectric.
The ground plane 12 and the EBG layer 13 have a self-inductance that is determined depending on their physical shapes. The power plane 11 and the EBG layer 13 have a predetermined capacitance that is determined depending on the gap between the cells, the permittivity of the fill material, and the size of the cells. A stopband center frequency of the EBG structure 10 changes depending on the self-inductance and the capacitance. Specifically, it is known that the stopband center frequency is proportional to √{square root over (L/C)}. Therefore, the EBG structure 10 can set the desired suppression band as the stopband by determining the L/C ratio. As the capacitance increases, the stopband width increases.
In this manner, by adjusting the gap, permittivity, and size of the cells of the EBG layer 13, the bandstop filter having stopbands of different center frequencies and different bandwidths can be implemented using the EBG structure 10. For example, when the cell size is small, the capacitance becomes small and the frequency of the stopband becomes high. In this case, however, the stopband width becomes narrow. When the stopband width has to be large for a small size of the package or board, the conventional EBG structure 10 cannot properly meet such a requirement.
Double-stacked EBG structures have also been proposed, to provide a broader effective stopband bandwidths. In such double-stacked EBG structures, the individual EBG layers can be designed to achieve different center frequencies and stopbands. More particular, two different cell sizes can be used for the two EBG layers, and alternatively or additionally different permittivity dielectric layers can be used for the respective EBG layers to achieve different center frequencies and stopbands. However, the proposed structures have the disadvantage of limiting the size variations of the cells due to accommodating respective vias for connection of the cells to the power the ground planes in gaps between the cells. As a result, the proposed designs are limited by low start frequencies for the effective stopband bandwidths. Furthermore, design variation based on using different permittivity dielectric layers has the disadvantage of having to integrate multiple dielectrics with different mechanical and processing properties.
A need therefore exists to provide an alternative double-stacked EBG structure that seeks to address at least one of the above problems.